FIG. 1 shows a conventional AC-DC power supply circuit. An AC power supply can be converted to DC input voltage Vg through a rectifier bridge and a filter capacitor, and then DC input voltage Vg can be converted to a DC voltage through a boost circuit and a flyback converter. The boost circuit may have a power factor correction (PFC) function. A PFC controller can receive DC input voltage Vg and bus voltage VB to generate current reference I*, and a current loop controller can control the switching operation of switch Q1 according to current reference I* and detected input current. The DC input voltage Vg may be converted to bus voltage VB, and the input current can be maintained in a same phase with the input voltage, thus improving the circuit power factor.